Fix broken tag autocomplete on multiple pages.

Mark all tag <input>s with a `data-autocomplete` attribute, instead of
hardcoding a list of html IDs to autocomplete in javascript.

This way should be less error prone. It fixes autocomplete in several places:

* Autocomplete for the search box on /posts didn't work in the
  responsive layout. This was because /posts has two search boxes that
  both have the id `tags`: one in the normal sidebar, and one in the
  responsive tag list. $("#tags") only initialized autocomplete on the
  first one.

* Autocomplete didn't work on the aliases or implications pages. This
  was due to selecting the wrong html ids.

app/views/posts/partials/index/_mode_menu.html.erb

@@ -25,6 +25,6 @@
         <% end %>
       </select>
     </form>
-    <input id="tag-script-field" placeholder="Enter tag script" style="display: none; margin-top: 0.5em;"></input>
+    <input id="tag-script-field" data-autocomplete="tag-edit" placeholder="Enter tag script" style="display: none; margin-top: 0.5em;"></input>
   </section>
 <% end %>